Detergent additives with excellent properties have been previously disclosed, including copolymers having structural units derived from acrylic acid (salt), maleic acid (salt) and monomers containing a sulfonic acid (salt) free of ester bonds and amido bonds such as 3-allyloxy-2-hydroxypropanesulfonic acid (salt) commonly referred to as HAPS; copolymers having structural units derived from acrylic acid (salt), maleic acid (salt), and sulfoethyl methacrylate, referred to as SEMS; and copolymers having structural units derived from acrylic acid (salt), maleic acid (salt), and 2-acrylamido-2-methylpropanesulfonic acid (salt) referred to as AMPS. See Japanese Kokai patent Application Nos. 2000-355615 and Hei 3[1991]-234713.
With regard to the performance of detergent additives, in order to improve detergency, it is believed to be important to improve the clay dispersibility and the iron precipitation inhibition. See Japanese Kokai patent Application No. 2004-189785. However, when the copolymer of SEMS and AMPS is used as a detergent additive, there is the problem of hydrolysis of ester bonds and amido bonds, and sufficient performance cannot be displayed. Consequently, in order to improve the clay dispersibility and the iron precipitation inhibition, it is preferred that the copolymer having structural units derived from acrylic acid (salt), maleic acid (salt), and monomers containing a sulfonic acid (salt) free of ester bonds and amido bonds be used.
However, for the copolymer having structural units derived from acrylic acid, maleic acid, and HAPS, the molecular weight has typically been low, with weight-average molecular weights of 9000 or less, and the clay dispersibility, especially the clay dispersibility in water with high hardness, is not sufficiently satisfactory.
Also, a copolymer having a weight-average molecular weight of 36,000 and has structural units derived from acrylic acid, maleic acid, and HAPS has further been disclosed in Japanese Kokai Patent Application No Hei 2[1990]-129020. However, the proportion of structural units derived from maleic acid (salt) is small (10 mol % in Application Example 8), and the iron precipitation inhibition cannot be sufficiently displayed.
Further, a copolymer that has structural units derived from acrylic acid, and HAPS, and structural units displaying hydrophobicity is disclosed in Japanese Kokai Patent Application No. 2007-231263. However, as it does not have structural units derived from maleic acid (salt), it is impossible to have a sufficiently satisfactory iron precipitation inhibition. Here, for the copolymer having structural units derived from acrylic acid (salt), structural units derived from maleic acid (salt), and structural units derived from HAPS, in order to increase the weight-average molecular weight and to increase the proportion of structural units derived from maleic acid (salt), the method described in Patent Reference 1 may be adopted. However, in this case, the quantity of residual maleic acid (salt) is too large, and the iron precipitation inhibition is poor. This is undesired.